A jazz thing; a trio of organ, electric guitar, and drums - or a tenor saxophone in lieu of guitar. The organist plays the bass parts on pedals. Also the nucleus of larger ensembles - maybe other horns are involved, or guitar and sax co-exist. A populist endeavour - organists like Jimmy Smith and Jack McDuff "held down the fort" against those evil 60s free jazzers and their art-music aspirations; nowadays it's a neo-con thing in the hands of a Joey DeFrancesco.